1. Field of the Invention
This invention generally relates to the regeneration of adsorbent materials in a supercritical water medium. More particularly, the invention relates to the regeneration of the adsorptive capacity of activated carbon particles using supercritical water. Additionally, the invention relates to a process and apparatus for removing organic contaminants from activated carbon particles and for oxidizing the organic contaminants.
2. Description of the Related Art
The use of water and air streams during a variety of industrial processes tends to produce effluent streams contaminated with impurities. These impurities typically include both volatile and non-volatile organic compounds. Potable water (i.e., drinking water) may also become contaminated with organic contaminants. Before these contaminated streams may be used the organic impurities are typically removed. The removal of these impurities may be accomplished by the use of adsorbent particles. Activated carbon is a common adsorbent that tends to be used to purify contaminated effluent streams.
In a typical adsorptive process activated carbon particles are formed into a bed. Typically, granular activated carbon (GAC) having an average size greater than about 0.25 mm is used to form the bed. The contaminated stream to be treated is passed through the bed. Impurities in the stream tend to adhere to the surface of the activated carbon particles and become trapped within the pores of the activated carbon particles. Eventually, the activated carbon particles may become saturated with these impurities. When the activated carbon particles become saturated in this manner the adsorptive capacity of the activated carbon particles is reached and the particles may no longer remove impurities from the fluid stream. The activated carbon particles may then be replaced by unused activated carbon particles, or the activated carbon particles may be regenerated and reused.
Activated carbon particles (PAC) having an average size of less than 150 .mu.m may also be used to purify contaminated fluid streams. The activated carbon particles are typically suspended in the stream for a time sufficient to allow removal of the organic contaminates from the stream. Eventually the particles become saturated with impurities such that the particles may no longer remove impurities from the fluid stream. It is usually more economical to regenerate and reuse activated carbon particles rather than replace activated carbon particles with new material. The regeneration of activated carbon particles, especially GAC, involves the removal of the adsorbed impurities from the activated carbon. A typical method of regenerating activated carbon particles involves a series of heating steps. Initially, the activated carbon particles are dried. The dried activated carbon particles may undergo a thermal desorption step in which they are heated to temperatures from about 100.degree. C. to about 400.degree. C. A pyrolysis of any remaining organics may then be accomplished by heating the activated carbon particles to temperatures from about 200.degree. C. to about 650.degree. C. Finally, the pyrolysis residues remaining from the adsorbed organics are gasified by treatment with water, carbon dioxide and oxygen at a temperature greater than about 650.degree. C.
This high temperature purification process tends to cause the adsorbed impurities to vaporize and pass from the activated carbon particles. Heating in air or any sort of oxidizing atmosphere, however, may cause partial combustion of the activated carbon particles. This combustion may lead to an increase in the size of the pores in the activated carbon particles. As a result, the heat-regenerated activated carbon particles tend to have a lower adsorptive capacity.
In general, a heat process as described above, may only be used on granular activated carbon particles. This makes the process less applicable to activated carbon particles having an average size of less than about 150 .mu.m. Additionally, this type of process typically requires a large amount of energy in order to sustain the required temperatures throughout a continuous process. Finally, the equipment involved to run this type of process may prevent on site purification of activated carbon particles. This may require the transportation of contaminated activated carbon particles to a central recycling site. The transportation of the particles to and from a central recycling site tends to add to the cost of recycling the activated carbon particles.
Steam regeneration has also been used to partially regenerate the adsorptive capacity of activated carbon particles. Steam regeneration is typically effective in removing only a portion of the adsorbed impurities, leaving behind a substantial amount of non-volatile organic impurities (i.e., organic compounds with relatively high boiling points) adsorbed onto the activated carbon particles. Superheated steam (e.g., steam at temperatures approaching 300 to 500.degree. F. (150 to 260.degree. C.)) may also be used to partially regenerate the adsorptive capacity of activated carbon particles. Superheated steam tends to remove a greater portion of the adsorbed impurities, but also tends to leave impurities adsorbed upon the activated carbon particles. The impurities left adsorbed upon the activated carbon particles may be removed by further heating of the activated carbon under vacuum conditions. The use of steam for regenerating activated carbon particles will typically produce a substantial amount of water contaminated with organics. The impurities within this water usually need to be removed prior to introducing the water back into the environment.
A process which avoids some of these problems involves the use of a wet oxidation procedure, as described in U.S. Pat. No. 4,749,492 to Berrigan, Jr. et al. An aqueous mixture of activated carbon particles is treated with an oxidant at a temperature of about 475.degree. F. (245.degree. C.) and a pressure of about 900 psig. (62 bar). During this treatment a portion of the organic contaminants may be removed from the activated carbon particles. At the temperature and pressure of the reactor a portion of the organic contaminants may be oxidized such that the organic contaminants are no longer adsorbed by the activated carbon particles. The oxidized organic contaminants in the aqueous phase are separated from the activated carbon particles and the particles may be reused. Under the strong oxidizing conditions of such a process a substantial amount of the activated carbon particles tend to be oxidized. The oxidation of the activated carbon particles tends to lower the adsorptive capacity of the regenerated activated carbon.
It would therefore be desirable to have a more efficient process and apparatus by which impurities could effectively be removed or extracted from activated carbon particles with less degradation of the activated carbon. The process and apparatus would also optimally address the disposal and/or destruction of the separated organic contaminants.